Thin-film semiconductor device and apparatus for fabricating thin-film semiconductor device

ABSTRACT

There is provided a method of fabricating a thin-film semiconductor device, including the steps of (a) melting and recrystallizing at least a surface of a thin semiconductor film formed on a substrate, in a pressure lower than an atmospheric pressure or in inert gas atmosphere, (b) keeping the substrate in atmosphere including oxygen gas, and (c) forming an insulating film on the thin semiconductor film with the substrate being kept in a pressure lower than an atmospheric pressure or inert gas atmosphere.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of fabricating a thin-filmsemiconductor device and an apparatus for fabricating the same, and moreparticularly to a method of forming a thin silicon film used for acrystalline silicon thin-film transistor, and an interface betweensemiconductor and an insulating film, used for a field effecttransistor, and further to an apparatus of fabricating such a thinsilicon film and such an interface.

2. Description of the Related Art

Japanese Patent Publication No. 7-118443 has suggested a methodincluding the step of radiating laser beam having a short wavelength, toan amorphous silicon thin film formed on an amorphous substrate, tothereby fabricate a thin film transistor. The method makes it possibleto crystallize amorphous silicon without wholly heating the substrate,and hence, makes it possible to fabricate a semiconductor device or asemiconductor integrated circuit on a substrate having a wide area suchas a substrate to be used for a liquid crystal display, or a cheapsubstrate such as glass.

Japanese Unexamined Patent Publication No. 9-320961 has suggested amethod of fabricating a thin-film transistor. In this method, the stepsof forming an amorphous silicon thin film to which laser beam is to beradiated, radiating laser beam to the amorphous silicon thin film,carrying out hydrogenation in plasma, and forming a gate insulating filmare carried out in this sequence or other sequences without exposure toatmosphere.

The Publication also discloses an apparatus of fabricating asemiconductor thin film, including a first chamber in which a substrateis loaded in vacuum, a second chamber in which silicon is formed, athird chamber in which laser beam is radiated, a fourth chamber in whichan insulating film is formed, a fifth chamber in which annealing iscarried out in hydrogen atmosphere, a sixth chamber in which a substrateis unloaded, and a seventh chamber through which a substrate istransferred to other chambers.

A glass substrate is transferred into the apparatus from the firstchamber. The glass substrate can be transferred to any one of thechambers through the seventh chamber and a vacuum valve. The first toseventh chambers are equipped with a gas exhaust system independently ofone another, and hence, can exhaust reactive gas, inert gas and othergases introduced from gas introducers in the steps of forming a siliconfilm, forming an insulating film, and annealing.

When a substrate is transferred out of any one of the chambers, thechamber is sufficiently exhausted. When the chamber has almost the samepressure as that of the seventh chamber, a vacuum valve is released, andthen, a substrate is taken out of the chamber by means of a robot. Then,the vacuum valve is closed.

The substrate is introduced into a next chamber having almost the samevacuum degree as that of the seventh chamber after releasing a vacuumvalve, transferring the substrate into the third chamber, and closingthe vacuum valve. After closing the vacuum valve, a process gas isintroduced into the chamber, and a pressure and a temperature in thechamber are adjusted to a predetermined pressure and temperature. Then,laser beam is radiated to the substrate.

The substrate is transferred among the first to sixth chambers in such amanner as mentioned above. A plurality of substrates can be transferredamong the chambers through the use of a plurality of robots, in whichcase, the chambers are sufficiently exhausted. A vacuum valve may bereleased and closed, and the substrate may be transferred after thechambers are caused to have almost the same pressure in inert gas,nitrogen gas or hydrogen gas atmosphere.

The substrate is transferred between the first chamber and atmosphereand further between the sixth chamber and atmosphere, after nitrogen orinert gas is leaked with the vacuum valve being closed, and a valve isreleased to thereby allow the first and sixth chambers to be in fluidcommunication with atmosphere.

Thus, all the steps are carried out without exposure to atmosphere. Thereason is as follows. Since a surface of silicon formed by lasercrystallization is quite active, contaminants are likely to be adheredto the surface, if the silicon is exposed to atmosphere. This results indegradation in performances of resultant TFT. As an alternative, thereis dispersion among performances of TFT. In order to avoid suchdegradation or dispersion, all the steps are carried out withoutexposure to atmosphere.

The inventor conducted the experiment in which excimer lasercrystallization and formation of a silicon dioxide film were carried outin the same apparatus in two cases in one of which a substrate wasexposed to atmosphere and in the other of which a substrate was notexposed to atmosphere. Herein, “the same apparatus” includes a case inwhich a substrate is transferred to another apparatus without exposureto atmosphere. In the case in which a substrate was not exposed toatmosphere, a fabrication yield was much enhanced because dusts andparticles were prevented from adhering to a product. However, it wasalso found out that such enhancement in a fabrication yield can beobtained by enhancing cleanness in a clean room. A fabrication yield wasenhanced best in an film-forming apparatus including a washing devicetherein.

Comparing a trap level density in a silicon film to an interface leveldensity (or density of electric charge in a fixed oxide film), the traplevel density is obviously greater than the interface level density.That is, there is a problem of insufficiency in performances of asilicon film (or a trap level density) in order to have sufficientcleanness in a product having a silicon film and a gate insulating filmboth formed without exposure to atmosphere in the same apparatus.

The inventor analyzed the above-mentioned problem, and resultingly,found out the following problems in connection with steps of fabricatinga silicon film and a gate insulating film and an apparatus offabricating the same.

The first problem is as follows. For instance, in a cluster tool typeapparatus suggested in Japanese Unexamined Patent Publication No.7-99321, a plurality of chambers is arranged having its own purpose.Hence, it is quite difficult to keep a chamber located at a core, awayfrom contaminants. There occurs cross-contamination in transfer of asubstrate between chambers, even though the cross-contamination isslight.

The second problem is as follows. For instance, an in-line typeapparatus suggested in Japanese Unexamined Patent Publication No.5-182923 is accompanied with a problem that it is unavoidable generationof minute dust, in particular, metal particles due to a great frictionalarea between parts in vacuum.

The third problem is as follows. Silicon crystallized by laser beamwould have a quite active surface. For instance, if silicon is coatedwith active species having energy, for instance, radical species such ashydrogen radical, oxygen radical, hydrogen ion, oxygen ion, ion speciesor ozone, after the silicon has been crystallized by laser beam, butbefore a gate insulating film is formed, contaminants adhered to a wallof a chamber and metal constituting a wall of a chamber are excited,resulting in that atmosphere in which a substrate is put iscontaminated.

The fourth problem is that since laser radiation in oxidation atmospherereflects dispersion in laser intensity in a step of introducing oxygeninto silicon, there would be dispersion in a concentration of oxygen ina silicon film. This results in non-uniformity in characteristics ofresultant silicon films.

The fifth problem is as follows. When a plurality of steps are to besuccessively carried out without exposure to atmosphere, for instance,steps of crystallizing a silicon film by means of laser beam andthereafter forming a gate insulating film, though it is possible toreduce contaminants adhered to the silicon film by not exposing thesilicon film to atmosphere, the above-mentioned problems still interferewith fabrication of a semiconductor device. A conventional method offabricating a semiconductor device such as LSI includes a step ofcarrying out thermal oxidation at about 1000 degrees centigrade to forman interface in a crystalline silicon film. This means that it isnecessary to control contaminants even in a vacuum apparatus.

There has been suggested remote plasma chemical vapor deposition (CVD)to reduce damage caused by plasma and form a qualified gate insulatingfilm. For instance, Japanese Unexamined Patent Publication No. 5-21393has suggested a plasma CVD apparatus including a first chamber in whichplasma is generated and a second chamber in which a substrate isprocessed. The first chamber is separate from the second chamber. It isconsidered that the suggested apparatus would accomplish a low densityof electric charge of a fixed oxide film in the range of 1×10¹¹ to1×10¹²cm⁻², and a low interface level density smaller than 6×10¹⁰cm⁻²eV⁻². However, this advantage of accomplishing such low densities isrestricted to performances of a silicon film.

The sixth problem is as follows. A chamber in which a substrate isprocessed is frequently caused to have a high vacuum degree or a lowpressure in order to prevent contaminants from adhering to a surface ofa substrate. In particular, in plasma CVD where a film is to be formedat a pressure smaller than atmospheric pressure, a chamber is exhaustedalmost to an ultimate vacuum degree except while a film is being formed.For the same reason, a chamber through which a substrate is transferredto another chambers, in a batch-type apparatus, is exhausted almost toand kept at an ultimate vacuum degree.

In an excimer laser radiation apparatus, excimer laser beam is oftenradiated in vacuum atmosphere. However, silicon particles separated froma silicon film during laser radiation are adhered to a window throughwhich laser beam is introduced into the apparatus, resulting inreduction in a laser transmission rate with the lapse of time.

In order to solve this problem, Japanese Unexamined Patent PublicationNo. 9-139356 has suggested a laser annealing apparatus in which excimerlaser beam is focused on a window to which silicon is adhered, tothereby thermally decompose silicon. However, in this apparatus, since alaser beam which is usually focused on a surface of a substrate isfocused on a surface of the window having a different optical length, itwould be absolutely necessary to rearrange optical systems. When thereare employed optical systems having a small focal length, in particular,when mask projection method is carried out, it is necessary toaccurately position optical systems, resulting in reduction in anoperation efficiency of the apparatus.

Japanese Unexamined Patent Publication No. 3-292719 has suggested amethod of forming a silicon semiconductor layer, including the steps offorming a silicon semiconductor layer on an insulating substrate at atemperature equal to or lower than 600 degrees centigrade, and radiatingenergy beam to the silicon semiconductor layer to thereby turn thesilicon semiconductor layer into polysilicon.

Japanese Unexamined Patent Publication No. 9-36376 has suggested amethod of fabricating a thin-film semiconductor device, including thesteps of forming a thin semiconductor film on an insulating substrate,the thin semiconductor film containing hydrogen at 10% or greater aswell as inert impurities, annealing the insulating substrate at 350degrees centigrade or higher to thereby remove hydrogen such that thethin semiconductor film contains hydrogen at 10% or smaller, andradiating laser beam to the thin semiconductor film to thereby activatethe impurities.

Japanese Unexamined Patent Publication No. 5-326397 has suggested amethod of fabricating a semiconductor device, including the steps offorming an amorphous semiconductor film on a semiconductor substrate atsuch a temperature that the amorphous semiconductor film has a planarsurface at a pressure smaller than an atmospheric pressure, annealingthe semiconductor substrate in inert gas atmosphere at a temperaturehigher than a temperature at which the amorphous semiconductor film isformed, to thereby turn the amorphous semiconductor film into apolysilicon film having irregularities at a surface thereof, thepolysilicon film acting as a first electrode of a capacitor, forming adielectric film on the first electrode, and forming a second electrodeon the dielectric film.

Japanese Unexamined Patent Publication No. 5-182919 has suggested amethod of fabricating a thin polysilicon film, including the steps offorming an amorphous silicon film on a glass substrate by LPCVD, puttingthe substrate in an oxygen atmosphere to thereby oxidize a surface ofthe amorphous silicon film, and annealing the amorphous silicon film ininert gas atmosphere at 600 degrees centigrade or smaller to therebyturn the amorphous silicon film into polysilicon.

Japanese Unexamined Patent Publication No. 11-17185 has suggested amethod of fabricating a liquid crystal display device, including thesteps of forming a semiconductor film almost all over a substrate todefine an insulating gate type transistor, heating and recrystallizingthe semiconductor film, forming a gate insulating film of the insulatinggate type transistor on the thus recrysallized semiconductor film, andforming a gate electrode of the insulating gate type transistor almostall over the gate insulating film. All of the steps are carried out inan apparatus which is kept vacuous.

Japanese Unexamined Patent Publication No. 10-149984 has suggested amethod of forming polysilicon, including the steps of radiating laserbeam to an amorphous silicon film formed on a substrate, in ahermetically sealed chamber, and annealing the amorphous silicon film tothereby turn the film into polysilicon. The chamber is designed to havea vacuum degree of 0.1 Torr or higher, and have atmosphere of at leastone of hydrogen, nitrogen and inert gas.

Japanese Unexamined Patent Publication No. 10-116989 has suggested amethod of fabricating a thin-film transistor, including the steps offorming a semiconductor film on a substrate without exposure toatmosphere, crystallizing the semiconductor film in non-oxidizingatmosphere without exposure of the substrate to atmosphere, forming afirst gate insulating film on the semiconductor film without exposure ofthe substrate to atmosphere, annealing the first gate insulating filmand the semiconductor film, patterning the first gate insulating filmand the semiconductor film, hydrogenating the substrate, and forming asecond gate insulating film on the first gate insulating film.

Japanese Unexamined Patent Publication No. 9-17729 has suggested amethod of fabricating a semiconductor device, including the steps offorming a first insulating film, a thin amorphous semiconductor film,and a second insulating film on an upper surface of an insulatingsubstrate without exposure to atmosphere, and radiating laser beamthrough a lower surface of the insulating substrate to thereby turn thethin amorphous semiconductor film into crystal.

Japanese Unexamined Patent Publication No. 9-7911 has suggested anapparatus for fabricating a semiconductor device, including (a) a laserannealing unit having a chamber in which a substrate is kepthermetically sealed, and in which laser beam is radiated to thesubstrate, (b) a film-forming unit having a chamber in which a substrateis kept hermetically sealed, and in which a thin film is formed on thesubstrate, and (c) a transfer unit which transfers the substrate betweenthe chambers with the substrate being kept hermetically sealed.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems, it is an object of the presentinvention to provide a method and an apparatus both of which is capableof fabricating a thin silicon film having a low trap level density, byradiating laser beam thereto.

It is also an object of the present invention to provide a method and anapparatus both of which are capable of providing an interface betweensemiconductor and an insulating film which interface has a smallinterface level density, that is, a field effect transistor havingimproved characteristics.

Another object of the present invention is to provide a method and anapparatus both of which are capable of forming the above-mentioned thinsilicon film at a temperature in the range of room temperature to 600degrees centigrade for the purpose of allowing to use a cheap glasssubstrate.

In one aspect of the present invention, there is provided a method offabricating a thin-film semiconductor device, comprising the steps of(a) melting and recrystallizing at least a surface of a thinsemiconductor film formed on a substrate, in a pressure lower than anatmospheric pressure or in inert gas atmosphere, (b) keeping thesubstrate in atmosphere including oxygen gas, and (c) forming aninsulating film on the thin semiconductor film with the substrate beingkept in a pressure lower than an atmospheric pressure or inert gasatmosphere.

It is preferable that the atmosphere predominantly includes oxygen gasor the atmosphere includes only oxygen gas.

It is know that if silicon is left in atmosphere, there would be formeda natural oxidation film on an active surface of silicon. Since organicsubstances and/or metal particles floating in the air are absorbed intothe natural oxidation film, the natural oxidation film formed inatmosphere is not suitable for formation of a clean interface, which isan object of the present invention.

In a conventional method of fabricating a bipolar transistor, in orderto form silicon crystal by epitaxial growth, the following steps arecarried out: removing a natural oxidation film through the use ofhydrofluoric acid, forming a chemical oxide film through the use ofheated solution of ammonia/H₂O₂/H₂O or HCl/H₂O₂/H₂O, annealing at 1000degrees centigrade or greater in an epitaxial growth furnace through theuse of hydrogen gas, to thereby remove the chemical oxide film andresultingly form a clean surface of silicon, and growing a film.

However, when a step which is to be carried out at a temperature in therange of room temperature to 600 degrees centigrade has to be carriedout, the high-temperature step in the above-mentioned conventionalmethod can not be selected.

In addition, since a surface of a thin silicon film having beencrystallized by laser beams has experienced a temperature of 1000degrees centigrade or higher at which silicon would be molten, even inan order of nanoseconds, the surface is in quite active condition.Hence, even in a vacuous chamber, contaminants would be readily adheredto the surface, if atmosphere in the chamber is suitably controlled.

In contrast, in accordance with the present invention, highly purifiedoxygen gas is introduced into a chamber just after silicon has beencrystallized by laser beams, to thereby form a natural oxidation filmhaving a low concentration of contaminants. By forming such a naturaloxidation film on a surface of silicon, it would be possible to preventcontaminants from being adhered to a surface of silicon in variouschambers such as a chamber in which laser beam is radiated, a chamberthrough which a substrate is transferred to another chamber, or achamber in which a film is formed.

If active gases such as radicals or ions are used for formation of anatural oxidation film, it would be possible to effectively form anatural oxidation film and establish hydrogen passivation. However, theuse of those active gases might cause absorption of contaminants adheredto a wall of a chamber into a natural oxidation film, and thus, it isnot preferable to use such active gases.

The inventor conducted the experience in which silicon dioxide filmswere formed on a silicon wafer through the use of oxygen gases A, B andC having different purities, and then, leakage current was measured foreach one of oxygen gases A, B and C.

TABLE 1 Purity of Concentration of Concentration of O₂ Gas (%) CO andCO₂ CmHn Gas A 99.7 < 2 ppm < 33 ppm Gas B 99.99 < 1.5 ppm < 1 ppm Gas C99.9999 < 0.5 ppm < 0.3 ppm

There were obtained current densities X1, X2 and X3, when an electricfield of 5 MV/cm was applied to the samples in which the gases A, B andC were used, respectively. The relation among X1, X2 and X3 is X1>X2>X3.

Since carbon existing in a silicon dioxide film would cause currentleakage, it is necessary to reduce a concentration of carbon. Inaddition, since metals such as Na, K or Li exist as movable ions in anoxide film, they would cause a threshold value to shift. Accordingly,gas having a high purity, such as O₂, N₂O, silane or disilane isnecessary to be prepared for formation of a thin silicon film, oxidationof a surface of silicon or deposition of a silicon dioxide film.

Oxygen gas is fractionated from air through low temperature processing.In fractionation, hydrocarbon such as methane is all residual in oxygengas because a boiling point of methane is higher than a boiling point ofoxygen. Specifically, a boiling point of oxygen is −183 degreescentigrade, a boiling point of methane is −162 degrees centigrade, and aboiling point of nitrogen is −196 degrees centigrade. As a result,hydrocarbon in the air is condensed and residual in oxygen gas.

The following method is carried out in order to remove such hydrocarbon.First, porous catalyst such as Pt or Pd is heated. Then, hydrocarbon ismade to react with oxygen to thereby form CO₂ and H₂O. Those CO₂ and H₂Oare absorbed into an absorber. As a result, it is possible to reduce aconcentration of hydrocarbon in oxygen to 0.1 ppm or smaller, andconcentrations of CO₂ and CO to 0.1 ppm or smaller. Hence, an apparatusfor refining oxygen may be arranged upstream of a gas supplier whichsupplies gases to a process apparatus.

It is known that argon and nitrogen are residual in a step offractionation. However, highly purified argon or highly purifiednitrogen in an order of ppm does not cause any problems in the presentinvention. For instance, even if hydrogen, nitrogen or inert gas such asargon each having a purity of 99.9999% or higher is mixed with oxygengas, such mixture gas does not cause nay problems.

As a result of the experiments the inventor conducted, the following wasfound out.

It is preferable that the atmosphere includes oxygen gas having purityof 99.999% or greater.

It is preferable that the atmosphere further includes hydrogen gas.having purity of 99.999% or greater.

It is preferable that the atmosphere further includes nitrogen gashaving purity of 99.999% or greater.

It is preferable that the atmosphere further includes inert gas havingpurity of 99.999% or greater.

It is preferable that a process gas used for forming the insulating filmhas purity of 99.999% or greater.

It is preferable that a process gas used in the method includeshydrocarbon (CnHm) species having a total concentration of 1 ppm orsmaller.

It is preferable that recrystallization in the step (a) is carried outthrough laser radiation.

There is further provided a method of fabricating a thin-filmsemiconductor device, including the steps, in sequence, of (a)introducing a substrate into a vacuous chamber, (b) introducingnon-reactive gas into the chamber, (c) radiating laser beam to thesubstrate in the chamber, (d) introducing oxygen gas into the chamber,(e) exhausting the non-reactive gas and the oxygen gas until a pressurein the chamber is reduced down to a predetermined pressure, and (f)taking the substrate out of the chamber.

It is preferable that the non-reactive gas is selected from the groupconsisting of nitrogen gas, inert gas and hydrogen gas alone or incombination.

It is preferable that the method further includes the step (g) ofradiating laser beam to a window through which laser beam is radiatedinto the chamber such that laser beam is not radiated to a completedregion of the substrate, the step (g) being to be carried out betweenthe steps (d) and (e).

It is preferable that the non-reactive gas is kept introduced into thechamber to cause the non-reactive gas to have a constant pressure.

It is preferable that the method further includes the step of heatingthe substrate.

There is still further provided a method of fabricating a thin-filmsemiconductor device, including the steps, in sequence, of (a)introducing a substrate into a vacuous chamber, (b) introducingnon-reactive gas into the chamber, (c) radiating laser beam to thesubstrate in the chamber, (d) stopping introduction of the non-reactivegas into the chamber, (e) introducing oxygen gas into the chamber, (f)transferring the substrate from the chamber into a second chamber havingthe same internal pressure as that of the chamber.

It is preferable that the method further includes the steps of (g)radiating laser beam to a window through which laser beam is radiatedinto the chamber such that laser beam is not radiated to a completedregion of the substrate, after the substrate has been transferred intothe second chamber, and (h) exhausting the non-reactive gas and theoxygen gas until a pressure in the chamber is reduced down to apredetermined pressure.

In another aspect of the present invention, there is provided anapparatus for fabricating a thin-film semiconductor device, including(a) a first chamber which is capable of keeping the first chamber invarious pressure atmospheres, (b) an energy beam radiator which radiatesenergy beam to at least a surface of a semiconductor thin film formed ona substrate, (c) a carrier which carries the substrate between the firstchamber and a second chamber which is capable of accomplishing the samepressure atmosphere as that of the first chamber, (d) a firstgas-introducer which introduces nitrogen gas or inert gas into the firstchamber, (e) a gas pressure regulator which keeps the first and secondchambers in a predetermined pressure atmosphere, (f) a secondgas-introducer which introduces oxygen gas into the first chamber, and(g) a controller which controls operation of the energy beam radiator,the carrier, the first gas-introducer, the gas pressure regulator, andthe second gas-introducer.

It is preferable that the controller controls operation of the energybeam radiator, the carrier, the first gas-introducer, the gas pressureregulator, and the second gas-introducer such that the following stepsare carried out in this sequence: (a) adjusting pressures in the firstand second chambers so that the pressures are almost equal to eachother, (b) introducing the substrate into the first chamber from thesecond chamber, (c) introducing the nitrogen gas or inert gas into thefirst chamber, (d) radiating laser beam to the semiconductor thin film,and (e) introducing oxygen gas into the first chamber.

It is preferable that the apparatus further includes a heater forheating the substrate.

It is preferable that the controller controls the laser beam radiatorsuch that laser beam is radiated to the first chamber without radiatinglaser beam to a completed region of the substrate, after the oxygen gashas been introduced into the first chamber.

In order to successively carry out a step in which laser is radiated ata pressure almost equal to atmospheric pressure, a step of carrying outCVD at a vacuum degree of a couple of Torrs, and a step of transferringa substrate between those steps without exposure of the substrate toatmosphere, it is preferable to reduce a difference in a pressurebetween chambers between which the substrate is transferred.

After laser beam has been radiated in nitrogen or inert gas atmosphere,but before nitrogen or inert gas is compulsively exhausted, oxygen gasis introduced into a chamber in which a laser beam is radiated, tothereby oxidize an active surface of silicon. At this time, siliconadhered to a window through which laser beam is introduced into thechamber is also oxidized, because silicon has an active surface. Withoxygen being introduced into the chamber, the laser beam is introducedinto the chamber such that the laser beam is not radiated to effectiveareas.

In particular, when ultra-violet ray is used as a laser beam, oxygen gasis decomposed by the ultra-violet ray, and at the same time, siliconhaving been adhered to the window and having not been oxidized by oxygengas is heated. The thus decomposed, active oxygen reacts with the thusheated silicon to thereby form silicon dioxide, ensuring that reductionin a transmission rate of the laser beam is prevented.

The above-mentioned steps are carried out either in a condition in whichthe gas is sealed in the chamber or in a condition in which the gas isallowed to flow with the pressure being kept constant.

Then, the substrate is transferred from the chamber in which a laserbeam is radiated to the substrate to a second chamber through which thesubstrate is transferred to another chamber. When the substrate istransferred in a vacuum condition, nitrogen or inert gas is stopped tobe supplied into the chamber concurrently with stopping of introductionof oxygen gas and stopping of radiation of the laser beam, to therebymuch exhaust the gas.

After it has been confirmed that the chambers are in almost the samepressure atmosphere, a gate valve separating the chambers is made open,and the substrate is transferred from the previous chamber to the secondchamber.

When the substrate is transferred in oxygen atmosphere, the secondchamber is caused to have a predetermined pressure in advance by inertgas, nitrogen gas or oxygen gas all having a high purity, alone or incombination. Then, the chamber in which the laser beam is radiated iscaused to be in oxygen atmosphere, and the chamber is caused to have thesame pressure as that of the second chamber. Thereafter, the gate valveseparating the chambers from each other is made open, and the substrateis transferred to the second chamber.

Thus, it would be possible to prevent contaminants such as metal orcarbon from adhering to a surface of a silicon film formed on asubstrate, and prevent reduction in a transmission rate of laser beamthrough a laser-introducing window.

The above and other objects and advantageous features of the presentinvention will be made apparent from the following description made withreference to the accompanying drawings, in which like referencecharacters designate the same or similar parts throughout the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a timing chart of processing a substrate in a method inaccordance with the embodiment of the present invention.

FIG. 1B is another timing chart of processing a substrate in a method inaccordance with the embodiment of the present invention.

FIG. 2 is a flow-chart of processing a substrate in a method inaccordance with the embodiment of the present invention.

FIG. 3 is another flow-chart of processing a substrate in a method inaccordance with the embodiment of the present invention.

FIG. 4 is a cross-sectional view of an apparatus of forming a thin-filmsemiconductor device in accordance with the embodiment of the presentinvention.

FIG. 5 is a cross-sectional view of another apparatus of forming athin-film semiconductor device in accordance with the embodiment of thepresent invention.

FIG. 6 is a top view of an apparatus of forming a thin-filmsemiconductor device in accordance with the embodiment of the presentinvention.

FIG. 7 is a perspective view of an apparatus of radiating excimer laserbeam.

FIG. 8 is a side view of an apparatus of radiating excimer laser beam.

FIG. 9 is a top view showing mask patterns used in an apparatus ofradiating excimer laser beam.

FIG. 10A is a timing chart showing an operation of a stage used in anapparatus of radiating excimer laser beam.

FIG. 10B is another timing chart showing an operation of a stage used inan apparatus of radiating excimer laser beam.

FIG. 11 is a cross-sectional view of a plasma-CVD chamber.

FIGS. 12A to 12I are cross-sectional views of a thin-film transistor,showing respective steps of a method of fabricating the same.

FIGS. 13A to 13I are cross-sectional views of a thin-film transistor,showing respective steps of another method of fabricating the same.

FIGS. 14A to 14I are cross-sectional views of a thin-film transistor,showing respective steps of still another method of fabricating thesame.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments in accordance with the present invention will beexplained hereinbelow with reference to drawings.

FIG. 1A is a timing chart of processing a substrate in a method inaccordance with the embodiment of the present invention, and FIG. 2 is aflow-chart of processing a substrate in a method in accordance with theembodiment of the present invention.

With reference to FIG. 2, a first chamber in which a laser beam isradiated to a substrate and a second chamber through which the substrateis transferred to another chamber are both exhausted, in step S100.

Then, the first and second chambers are checked as to whether pressuresP in the chambers are equal to or smaller than a predetermined pressureX, in step S110.

If the pressures P are not equal to or smaller than the predeterminedpressure X (NO in step S110), the first and second chambers areexhausted again, in step S100 If the pressures P are equal to or smallerthan the predetermined pressure X (YES in step S110), a substrate istransferred to the first chamber from the second chamber.

Then, a gate valve separating the first and second chambers from eachother is closed to thereby shut gas communication therebetween. Then,nitrogen, inert gas such as argon or hydrogen gas alone or incombination is introduced into the first chamber in step S130. That is,the steps are carried out in accordance with the timing chartillustrated in FIG. 1A, as follows.

It is preferable not to stop exhausting the gas to thereby keep thefirst chamber in a constant nitrogen pressure. As an alternative, avalve through which gas is introduced into the first chamber may beclosed when the first chamber reaches a predetermined pressure afterexhausting the gas or introducing gas thereinto.

Then, a stage on which the substrate is lying is moved to apredetermined position. Then, a laser beam is radiated to the substrate,after a pressure in the first chamber reaches to a predeterminedpressure, and a heater for heating the substrate reaches a predeterminedtemperature.

Then, a laser beam is radiated to a desired area of the substrate forrecrystallization by moving the stage or laser beam. Then, a laser beamis radiated to a window through which the laser beam is introduced intothe first chamber such that a laser beam is not radiated to an effectivearea of the substrate by moving the stage or through the use of alight-shutter equipped in a vacuum equipment.

Radiation of a laser beam to the window, introduction of oxygen gas intothe first chamber, and introduction of nitrogen gas into the firstchamber are stopped concurrently or in turn to thereby exhaust the gas.

Then, it is checked whether pressures P in the first and second chambersare equal to or smaller than a predetermined pressure Y, in step S140.

If the pressures P are not equal to or smaller than a predeterminedpressure Y (NO in step S140), the first and second chambers areexhausted again until the pressures P reach the predetermined pressureY. If the pressures P are equal to or smaller than a predeterminedpressure Y (YES in step S140), a gate valve between the first and secondchambers is made open, and the substrate is transferred to the secondchamber from the first chamber.

FIG. 1B is another timing chart of processing a substrate in a method inaccordance with the embodiment of the present invention, and FIG. 3 isanother flow-chart of processing a substrate in a method in accordancewith the embodiment of the present invention.

With reference to FIG. 3, a first chamber in which a laser beam isradiated to a substrate and a second chamber through which the substrateis transferred to another chamber are both exhausted, in step S200.

Then, the first and second chambers are checked as to whether pressuresP in the chambers are equal to or smaller than a predetermined pressureX, in step S210.

If the pressures P are not equal to or smaller than the predeterminedpressure X (NO in step S210), the first and second chambers areexhausted again, in step S200. If the pressures P are equal to orsmaller than the predetermined pressure X (YES in step S210), asubstrate is transferred to the first chamber from the second chamber.

Then, a gate valve separating the first and second chambers from eachother is closed to thereby shut gas communication therebetween. Then,nitrogen, inert gas such as argon or hydrogen gas alone or incombination is introduced into the first chamber in step S230. That is,the steps are carried out in accordance with the timing chartillustrated in FIG. 1B, as follows.

Then, a stage on which the substrate is lying is moved to apredetermined position. Then, a laser beam is radiated to the substrate,after a pressure in the first chamber reaches to a predeterminedpressure, and a heater for heating the substrate reaches a predeterminedtemperature.

Then, a laser beam is radiated to a desired area of the substrate forrecrystallization by moving the stage or laser beam. Then, introductionof nitrogen gas into the first chamber is stopped, and introduction ofoxygen gas into the first chamber is started.

Then, it is checked whether pressures P in the first and second chambersare equal to or smaller than a predetermined pressure Y, in step S240.

If the pressures P are not equal to or smaller than a predeterminedpressure Y (NO in step S240), the first and second chambers areexhausted again until the pressures P reach the predetermined pressureY. If the pressures P are equal to or smaller than a predeterminedpressure Y (YES in step S240), a gate valve between the first and secondchambers is made open, and the substrate is transferred to the secondchamber from the first chamber.

Then, a laser beam is radiated to a window through which the laser beamis introduced into the first chamber such that a laser beam is notradiated to an effective area of the substrate.

Radiation of a laser beam to the window, introduction of oxygen gas intothe first chamber, and introduction of nitrogen gas into the firstchamber are stopped concurrently or in turn to thereby exhaust the gas.

In the above-mentioned embodiments, oxygen gas, inert gas, nitrogen gasand hydrogen gas all have a purity of 99.9999%.

FIG. 4 is a side view of an apparatus of forming a thin semiconductorfilm in accordance with the first embodiment of the present invention.

The illustrated apparatus includes a plasma CVD chamber C2, a chamber C5in which a laser beam is radiated to a substrate, and a chamber C7through which the substrate is transferred between the chambers C2 andC5. The chamber C2 is connected to the chamber C7 through a first gatevalve GV2, and the chamber C5 is connected to the chamber C7 through asecond gate valve GV5. The substrate can be transferred from the chamberC2 to the chamber C5 through the chamber C7 in vacuum, inert gas,nitrogen, hydrogen or oxygen atmosphere and in a high vacuum degreecondition or in a low or high pressure condition without exposure of thesubstrate to atmosphere.

In the chamber C5, a substrate Sub0 lies on a substrate stage S5 bymeans of a chuck unit (not illustrated). The substrate stage S5 can beheated to about 400 degrees centigrade.

In the chamber C2, the substrate Sub0 lies on a substrate holder S2which can be heated to about 400 degrees centigrade.

In the first embodiment, a thin silicon film Si1 is formed on the glasssubstrate Sub0. A laser beam is radiated to the thin silicon film Si1 inthe chamber C5, and resultingly, the thin silicon film Si1 is turnedinto a thin crystalline silicon film Si2. Then, the substrate Sub0 onwhich the thin crystalline silicon film Si2 lies is transferred into thechamber C2.

Laser beams emitted from first and second excimer lasers EL1 and EL2pass along first and second beam lines L1 and L2, and reach a surface ofthe substrate through an optical system Opt1 which synthesizes laserbeams to each other, a mirror Opt11, a transparent mirror Opt12, anoptical system Opt2 which radiates a laser beam to an object, ahomogenizer Opt 20, an optical mask Opt2 fixed to an optical mask stageOpt22, an optical apparatus Opt23 for projection, and a laserintroduction window W1.

Though the first embodiment is designed to include two excimer lasersEL1 and EL2, there may be used any number of excimer lasers. In place ofthe excimer laser, there may be used a CO₂ laser, a pulse laser such asYAG laser, or CW light source such as an argon laser.

In the plasma CVD chamber C2, a region D2 in which plasma is generated,defined by RF electrode D1 and a plasma-confinement electrode D3, ispositioned away from a region in which the substrate is located.

A silicon dioxide film can be formed on the substrate by introducing O₂and He gases and silane gas into the region D2 through a process gasintroducer D4.

The apparatus further includes a controller CL controlling operation ofthe above-mentioned parts constituting the apparatus.

For instance, the controller CL controls an operation of the apparatusby adjusting pressures in the chambers C2, C5 and C7 so that thosepressures are almost equal to one another, introducing the substrateinto the chamber C5 from the chamber C2 through the chamber C7,introducing nitrogen gas or inert gas into the chamber C5, radiating alaser beam to the thin silicon film, and introducing oxygen gas into thechamber C5.

FIG. 5 illustrates an apparatus of forming a thin semiconductor film, inaccordance with the second embodiment. In comparison with the apparatusin accordance with the first embodiment, illustrated in FIG. 4, theapparatus in accordance with the second embodiment further includes anoxidation chamber C5 a and a gate valve GV5 a through which theoxidation chamber C5 a is connected to the chamber C5.

The gate valve GV5 a allows the laser-introduction window W1 to be awayfrom the chamber C5. Oxygen and other gases can be introduced into thechamber C5 independently of one another. Hence, it is possible toradiate the laser beam to the laser-introduction window W1 withoutradiating the laser beam to the substrate, by radiating the laser beamin the oxidation chamber C5 a.

Since the gate valve GV5 a acts as a shutter to the substrate in thechamber C5, it would be possible to oxidize silicon adhering to asurface of the laser-introduction window W1 by radiating the laser beamto the laser-introduction window W1, for instance, during a period oftime in which the substrate waits to be taken out of the chamber C5. Oneor both of the beams emitted from the first and second excimer lasersEL1 and EL2 may be radiated to the chamber C5. It is also possible tolengthen a lifetime of expensive optical systems and radiate beamshaving a uniform intensity profile to the laser introduction window W bypartially or wholly removing the optical mask Opt21 and the projectionunit Opt23.

FIG. 6 is a plan view of the apparatus for forming a thin semiconductorfilm. The apparatus includes a central chamber C7 through which asubstrate is transferred to another chamber. The apparatus furtherincludes a loading and unloading chamber C1, a plasma CVD chamber C2, achamber C3 in which the substrate is heated, a chamber C4 in whichhydrogen plasma is generated, and a chamber C5 in which a laser beam isradiated to the substrate. These chambers C1 to C5 are connected to thecentral chamber C7 through gate valves GV1 to GV5, respectively.

Laser beams coming along first and second beam lines L1 and L2 areradiated onto a surface of the substrate through an apparatus Opt1 forsynthesizing laser beams to each other, an apparatus Opt2 for radiatinglaser beams, and a laser-introduction window W2.

Gas suppliers gas1 to gas7 and ventilation devices vent1 to vent7 areconnected to the chambers C1 to C7, respectively, for introducingdesired gas species, establishing a desired process pressure, exhaustinggases, and making a vacuum condition. Substrates sub2 and sub6 arehorizontally laid, as indicated with broken lines.

The apparatus further includes a controller CL which controls anoperation of other parts constituting the apparatus.

FIG. 7 is a perspective view of a chamber in which a laser beam isradiated to a substrate.

A pulsed ultra-violet beam emitted from first and second excimer lasersEL1 and EL2 is introduced into a homogenizer Opt2Oa through mirrors Opt3and Opt3 a, and lens Opt4. Though the illustrated chamber is designed tohave two excimer lasers as a laser source, it should be noted that thechamber may have any number of excimer lasers.

An optical mask Opt21 shapes the beam such that an intensity profile ofthe beam has desired uniformity, for instance, distribution of ±5% in aplane. Since an intensity profile and total energy of original beamsemitted from an excimer laser may vary at each of periods betweenpulses, it is preferable for the chamber to have a unit for uniformizinga beam intensity on the optical mask Opt 21. The homogenizer Opt2Oa isgenerally comprised of a fly-eye lens or a cylindrical lens.

The beams having an optical pattern formed by the optical mask Opt21 areradiated to a substrate Sub0 lying in a vacuum chamber C0 through astepper Opt23 a and a laser-introduction window W0.

The substrate lies on a substrate stage S0. A laser beam having apattern can be radiated to a desired region, for instance, a region ex0to which a pattern is to be transferred, by moving the substrate stageS0. Though FIG. 7 illustrates an optical system for projection in areduced scale, there may be used an optical system for projection in anequal scale or in an increased scale.

A laser beam may be radiated to a desired region in the substrate bymoving the substrate stage S0 in X-Y plane. The above-mentioned opticalmask is fixed on a mask stage (not illustrated). It is also possible toradiate a laser beam to a desired region in the substrate by moving themask stage.

Hereinbelow is explained an example of a method of radiating a laserbeam having a desired pattern, to a substrate in desired conditions. Itis necessary to effect slight adjustment in order to properly set anoptical axis. Hence, hereinbelow is explained a method of adjusting aposition of a substrate, after an optical axis has been properly set.

A position of a surface to an optical axis is determined by amending aposition of the surface in a focal direction or Z-axis direction andperpendicularity to an optical axis. Hence, perpendicularity to anoptical axis is amended by adjusting a direction θxy in which tilting isamended, a direction θxz in which tilting is amended, and a directionθyz in which tilting is amended, among the direction θxy, the directionθxz, the direction θyz, a direction X in which a region to be exposed toa laser beam is moved, a direction Y in which a region to be exposed toa laser beam is moved, and a direction Z in which a focus is adjusted.In addition, a surface of a substrate to which a laser beam is to beradiated is positioned in accordance with a depth of focus of an opticalsystem, by adjusting the direction Z.

FIG. 8 is a side view of an apparatus for adjusting alignment of asubstrate. An optical mask Opt21, a stepper Opt23 a and alaser-introduction window W0 are arranged relative to an exposure axisL0, as illustrated in FIG. 8.

A substrate Sub0 is placed on a heater H0 having a unit of absorbing asubstrate thereto. The heater H0 is fixed on a substrate XYZθxyθxzθyzstage S0 a in a vacuum chamber C0. Though there is used a vacuumchamber, it is preferable that a laser beam is radiated to the substratein inert gas, hydrogen, oxygen or nitrogen atmosphere. A pressure insuch atmosphere may be almost equal to atmospheric pressure. By usingthe heater H0 having a unit of absorbing a substrate thereto, thesubstrate can be heated at a temperature in the range of roomtemperature to about 400 degrees centigrade while a laser beam isradiated to the substrate.

By equalizing a pressure of the atmosphere to about an atmosphericpressure, the substrate can be adsorbed to a vacuum chuck. This meansthat misregistration can be prevented even if the substrate stage ismoved in the chamber, and that the substrate can be fixed to thesubstrate stage, even if the substrate is curved or deformed. Inaddition, it is also possible to minimize deviation of a depth of focus,caused by curvature and deformation of the substrate which occur whenheated.

Laser interferometers i1 and i2 measures alignment of a substrate and aposition of a substrate in a Z-axis direction through a measurementwindow W-I and a measurement mirror Opt-i. Alignment of a substrate ismeasured by observing an alignment mark formed on a substrate, by meansof an off-axis microscope, a light source Lm for a microscope, and amicroscope device Opt-m. A desired position at which the substrate isexposed to a laser beam can be measured based on data indicative of aposition of the substrate, provided by the laser interferometers i1 andi2.

Though FIG. 8 shows an off-axis method, there may be used a through thelens method or a through the mask (or reticle) method. In addition, anerror in measurement may be standardized by determining a linercoordinate through method of least squares, based on a plurality ofmeasured positions.

FIG. 9 shows a positional relation between a mask pattern and analignment mark. A mask is comprised of a region M1 through which asubstrate is not exposed to a laser beam and a region M2 through which asubstrate is exposed to a laser beam.

For instance, when an excimer laser is used as a laser source, a metalfilm such as aluminum, chromium or tungsten, which absorbs aultra-violet ray and a dielectric multi-layered film which reflects aultra-violet ray are formed on a quartz substrate through which aultra-violet ray passes, and those layers are patterned through the useof photolithography and etching.

A silicon film is exposed to a ultra-violet ray in accordance with apattern, which is indicated as a white area in FIG. 9-A, and as aresult, there are formed exposed silicon areas Si2 in non-exposedsilicon areas Si1, as illustrated in FIG. 9-B. When a silicon film isexposed to a ultra-violet ray, it is possible to expose a predeterminedposition on a silicon film to a ultra-violet ray by aligning the mark M1of the mask with the mark M2 of the substrate before exposure of thesilicon film to a ultra-violet ray.

In a method of fabricating a thin-film transistor including theabove-mentioned thin silicon film, when it is necessary to position amask in the first step, that is, an alignment mark is not formed on amask, it would be possible to form an alignment mark making use of anoptical color difference between amorphous silicon and crystal silicon,by exposing an exposure mark M3 to a laser beam at the same time when athin silicon film is exposed to a laser film. Hence, it is possible toform a desired device such as a transistor in a desired area having beenreformed, by carrying out lithography in later steps on the basis of thethus formed alignment mark.

After a step of exposing the thin silicon film to a laser beam has beenfinished, a silicon dioxide film is formed on the thin silicon film, anda desired region in the silicon layer is etched for removal, asillustrated in FIG. 9-C.

The silicon film and the silicon dioxide film are etched for removal ina region Si3. Silicon dioxide films Si4 and Si5 are formed on thenon-exposed silicon area Si1 and the exposed silicon area Si2 inmulti-layered structure. By forming an island comprised of a siliconfilm covered with an oxide film, it is possible to form an alignmentmark necessary for alignment in forming channel/source/drain regions ofa thin film transistor and in later steps.

FIGS. 10A and 10B show examples 1 and 2 of a timing chart of anoperation.

In example 1, a substrate is moved to a desired position by an operationof a substrate stage. Then, a focus is adjusted and an alignment step iscarried out to thereby accurately position a substrate at apredetermined position at which the substrate is exposed to a laserbeam. For instance, the substrate is positioned at a predeterminedposition such that an error in positioning is within an allowable rangeof about 0.1 μm to 100 μm.

After positioning the substrate, a laser beam is radiated to thesubstrate. Then, the substrate is moved for next exposure. Then, a laserbeam is radiated to a predetermined region of the substrate. Then, thesubstrate is exchanged to a next substrate.

The same steps as the above-mentioned steps are carried out to the nextsubstrate.

In example 2, a substrate is moved to a desired position by an operationof a substrate stage. Then, a focus is adjusted and an alignment step iscarried out to thereby accurately position a substrate at apredetermined position at which the substrate is exposed to a laserbeam. For instance, the substrate is positioned at a predeterminedposition such that an error in positioning is within an allowable rangeof about 0.1 μm to 100 μm.

After positioning the substrate, a mask stage starts moving. In order toavoid dispersion in movement distance of the mask stage, a laser beam isradiated to the substrate after movement of the mask stage. By movementof the mask stage, the substrate is exposed to a laser beam at aposition away from an alignment position. Hence, an offset caused bythat has to be taken into consideration.

It is also possible to start driving a laser source earlier thanradiation of a laser beam to a substrate, and open a shutter to therebyradiate a laser beam to a substrate when an output intensity of thelaser source has stabilized. In particular, in a method where an excimerlaser is used as a laser source, and oscillation and stop of a laserbeam are repeated, first tens of pulses are known to be quite unstable.Hence, if it is intended to stop radiation of those unstable laserpulses, there may be selected a system where a laser beam is interruptedin accordance with movement of the mask stage.

Then, the substrate is moved for next exposure. Then, a laser beam isradiated to a predetermined region of the substrate. Then, the substrateis exchanged to a next substrate.

The same steps as the above-mentioned steps are carried out to the nextsubstrate.

A laser beam of 1 mm×50 μm was scanned to a thin amorphous silicon filmhaving a thickness of 75 nm, at a pitch of 0.5 μm. One laser source wasused. A laser radiation intensity was set at 470 mJ/cm² at a surface ofthe silicon film. As a result, there was obtained a thinmono-crystalline silicon film extending in a direction in which thelaser beam was scanned.

Then, a laser beam was radiated to the silicon film from a second lasersource at a laser radiation intensity of 150 mJ/cm² at a pitch of 1.0 μmwhen 100 nsec had passed after first radiation of a laser beam from thefirst laser source. As a result, there was obtained a thinmono-crystalline silicon film extending in a direction in which thesecond laser beam was scanned. The resultant crystal silicon film had atrap level density smaller than 1×10¹² cm⁻².

FIG. 11 illustrates the plasma CVD chamber C2. Electric power issupplied to a radio-frequency electrode RF2 from a radio-frequency powersource RF 1. Herein, it is preferable for the radio-frequency powersource RF1 to emit a radio-frequency of 13.56 MHz or higher. Plasma isgenerated between an electrode RF3 and the radio-frequency electrodeRF2. The electrode RF3 is formed with a plurality of slits through whichgas passes. Radicals generated by plasma passes through the slits of theelectrode RF3 to enter a space in which a substrate Sub2 is arranged.

Another gas is introduced into the chamber C2 through a gas supplier RF4without exposure to plasma. As a result, a thin film is formed on thesubstrate Sub2, after gas phase reaction has occurred.

A substrate holder S2 on which he substrate Sub2 is fixed can be heatedto a temperature in the range of room temperature to 500 degreescentigrade by means of a heater (not illustrated). As illustrated,oxygen radicals are made to react with silane gas for formation of asilicon dioxide film through the use of a ventilation unit Vent2, and agas supplier Gas2 including an oxygen line Gas21, a helium line Gas22, ahydrogen line Gas23, a silane line Gas24, a heilum line Gas25, and anargon line Gas26.

A silicon dioxide film was formed in the following conditions.

Temperature of a substrate: 300 degrees centigrade

Pressure: 0.1 Torr

RF power: 100 W

Silane flow rate: 10 sccm

Oxygen flow rate: 400 sccm

Helium flow rate: 400 sccm

There was obtained a silicon dioxide film having superiorcharacteristic, specifically, a density of electric charge of fixedoxide film of 5×10 cm⁻².

If a ratio of oxygen flow rate to silane flow rate is increased, itwould be possible to obtain a silicon dioxide film having a greaterdensity of electric charge of fixed oxide film.

Though FIG. 11 illustrates a parallel plate type RF plasma CVDapparatus, there may be selected a method in which plasma is not used,such as low pressure chemical vapor deposition (LPCVD) or atmosphericpressure chemical vapor deposition (APCVD). As an alternative, there maybe selected plasma CVD using micro-waves or electron cyclotron resonanceeffect.

Table 2 shows an example of gas species necessary for forming a thinfilm other than a silicon dioxide film by means of the plasma CVDapparatus illustrated in FIG. 11.

When a Si₃N₄ film is to be formed, there may be used N₂ gas or ammoniagas as a process gas, and Ar or SiH₄ (silane) gas as a carrier gas. Whena thin silicon film is to be formed, there may be used hydrogen andsilane gases. As an alternative, when a thin silicon film is to beformed, there may be used H₂ and SiF₄ gases as a process gas, and anargon gas as a carrier gas. It is also possible to treat a thin siliconfilm and a silicon dioxide film with hydrogen plasma.

The above-mentioned gases have such a purity as listed in Table 3.

TABLE 2 Formation Formation Formation Formation Hydro- of SiO₂ of Si₃N₄of Si of Si genation Gas 21 O₂ N₂ Gas 22 He Ar Ar Gas 23 H₂ H₂ H₂ Gas 24SiH₄ SiH₄ SiH₄ Gas 25 He Ar Ar Gas 26 SiF₄

TABLE 3 Gas Species Necessarily Preferably O₂ >99.999% >99.9999% He >99.9999% Ar >99.9999% >99.99995% SiH₄ >99.999% H₂ >99.9999% >99.99997%N₂ >99.999% >99.9999%

FIGS. 12A to 12I illustrates respective steps of a method of fabricatinga thin-film transistor by means of an apparatus for fabricating a thinsemiconductor film.

First, a glass substrate Sub0 is washed to remove organic substances,metals and particles therefrom. Then, a substrate cover film T1 and athin silicon film T2 are formed on the glass substrate Sub0, asillustrated in FIG. 12A.

The substrate cover film T1 is comprised of a silicon dioxide filmhaving a thickness of 1 μm and formed by LPCVD at 450 degrees centigradeusing silane and oxygen gases as a process gas. It is possible to whollycover a surface of the glass substrate Sub0 with the substrate coverfilm T1 by LPCVD. In place of LPCVD, there may be selected plasma CVD inwhich TEOS (tetraethoxy silane) and oxygen gases are used as a processgas, or plasma CVD in which silane and oxygen gases are used as aprocess gas.

The substrate cover film T1 is composed of a material which can preventdiffusion of impurities contained in the glass substrate such as a glasssubstrate having a reduced concentration of alkaline metal, or a quartzsubstrate having a polished surface, and hazardous to a semiconductordevice.

The thin silicon film T2 has a thickness of 75 nm and is formed at 500degrees centigrade by LPCVD in which disilane gas is used as a processgas. As an alternative, the thin silicon film T2 may be formed at 550 to600 degrees centigrade by LPCVD in which monosilane is used as a processgas, or at 500 degrees centigrade or lower by LPCVD in which high gradesilane such as trisilane is used as a process gas.

If the silicon film T2 is formed at 400 degrees centigrade or higher, aconcentration of hydrogen atoms contained in the silicon film T2 isequal to or smaller than 1 atomic %, and hence, it would be possible toprevent the silicon film from being roughened because of evaporation ofhydrogen while a laser beam is radiated to the substrate.

As an alternative, it is possible to form a thin silicon film having alow concentration of hydrogen atoms even by plasma CVD as illustrated inFIG. 11 or ordinary plasma CVD, by adjusting a temperature of thesubstrate, a ratio of hydrogen flow rate to silane flow rate, and aratio of hydrogen flow rate to SiF₄ flow rate.

Then, the substrate Sub0 is washed to remove organic substances, metals,particles and oxides therefrom. Thereafter, the substrate Sub0 isintroduced into the apparatus for forming a thin film, in accordancewith the present invention.

Excimer laser XeCl having a wavelength of 308 nm is radiated to the thinsilicon film T2 as a laser beam L0 to thereby reform the thin siliconfilm T2 to a thin crystal silicon film T2 a, as illustrated in FIG. 12B.Crystallization by laser is carried out in nitrogen or inert gasatmosphere having a purity of 99.9999% or greater and at a vacuum degreeof 700 Torr or greater.

After completion of laser radiation, an oxygen gas having a purity of99.999% or greater is introduced into the apparatus. The above-mentionednitrogen gas is used for preventing impurities from entering the thinsilicon film T2 a, prevent impurities such as hydrocarbon from reactingwith a ultra-violet ray around a laser introduction window, and preventimpurities such as hydrocarbon from sticking to a laser introductionwindow in burning.

The reason why crystallization is carried out at 700 Torr or greater isthat it is possible to prevent silicon evaporated by laser radiationfrom sticking to the laser introduction window.

After exhausting the gases, the substrate Sub0 is transferred to theplasma CVD chamber through the substrate-transfer chamber. Then, a firstgate insulating film T3 is formed at a substrate temperature of 350degrees centigrade, using silane, helium and oxygen gases as processgases, as illustrated in FIG. 12C. The first gate insulating film T3 iscomprised of a silicon dioxide film and has a thickness of 10 nm.Thereafter, the substrate is subject to hydrogen plasma treatment andannealing, if necessary.

The above-mentioned steps are carried out in the apparatus of forming athin film, in accordance with the present invention.

Then, the thin silicon film T2 a and the silicon dioxide film T3 arepatterned into an island by photolithography and etching, as illustratedin FIG. 12D. In the step of patterning the films T2 a and T3, it ispreferable to select etching conditions in which an etching rate of thesilicon dioxide film T3 is higher than an etching rate of the thinsilicon film T2 a.

As illustrated in FIG. 12D, the thin silicon film T2 a and the silicondioxide film T3 are patterned to have a stepped sidewall, which ensuresprevention of gate leakage and hence high reliability. As analternative, the thin silicon film T2 a and the silicon dioxide film T3may be patterned to have a tapered sidewall.

Then, the substrate Sub0 is washed to remove organic substances, metals,particles and oxides therefrom. Then, a second gate insulating film T4is formed covering the island therewith, as illustrated in FIG. 12E. Thesecond gate insulating film T4 is formed at 450 degrees centigrade byLPCVD in which silane and oxygen gases are used as process gases. Thesecond gate insulating film is comprised of a silicon dioxide film andhas a thickness of 30 nm.

As an alternative, the second gate insulating film T4 may be formed byplasma CVD in which TEOS and oxygen gases are used as process gases,APCVD in which TEOS and ozone are used as process gases, or plasma CVDas illustrated in FIG. 11.

Then, a N⁺ silicon film having a thickness of 80 nm and a tungstensilicide film having a thickness of 110 nm are formed as a gateelectrode. The N⁺ silicon film is comprised preferably of a crystalphosphorus-doped silicon film formed by plasma CVD or LPCVD.

Thereafter, the N⁺ silicon film and the tungsten silicide film arepatterned by photolithography and etching into a gate electrode T5, asillustrated in FIG. 12E.

Then, as illustrated in FIGS. 12F and 12H, impurities are implanted intoregions T6 and T6 a with the gate electrode T5 being used as a mask.When a CMOS type integrated circuit is to be fabricated,photolithography is also carried out to thereby form both n-channel TFTincluding a N⁺ region and a p-channel TFT including a P⁺ region. Theremay be selected ion doping where mass separation of impurity ions to beimplanted is not carried out, ion implantation, plasma doping or laserdoping. Impurities are implanted to the regions T6 and T6 a withoutremoval of the silicon dioxide film or after removal of the silicondioxide film in dependence on a use of a resultant device and/or amethod of implanting impurities.

Then, as illustrated in FIGS. 12G and 12I, interlayer insulating filmsT7 and T7 a are formed. After contact holes are formed throughout theinterlayer insulating films T7 and T7 a, a metal film is deposited overthe products. Then, the metal film is patterned into a metal wiringlayer T8 by photolithography and etching, as illustrated in FIGS. 12Gand 12I.

The interlayer insulating films T7 and T7 a are preferably TEOS oxidefilms, silica coating films, or organic coating films, because they canbe readily planarized. The contact holes are formed by photolithographyand etching. The metal wiring layer is composed preferably of aluminumor copper both having a low resistance, or an alloy predominantlycontaining aluminum or copper. As an alternative, the metal wiring layermay be composed of refractory metal such as tungsten or molybdenum.

Thus, there is fabricated a thin-film transistor having highperformances and reliability.

FIGS. 13A to 13I illustrate respective steps of a method of fabricatinga thin-film transistor. In this method, a substrate is designed to havean alignment mark, and a laser beam is radiated to the substrate inaccordance with the alignment mark.

First, a glass substrate Sub0 is washed to remove organic substances,metals and particles therefrom. Then, a substrate cover film T1 and atungsten silicide film are formed on the glass substrate Sub0. Then, thetungsten silicide film is patterned into an alignment mark T9 byphotolithography and etching. Then, a mark protection film T10 is formedover the substrate cover film T1 and the alignment mark T9 to protectthe alignment mark T9. Then, a thin silicon film T2 is formed on themark protection film T10, as illustrated in FIG. 13A.

When a laser beam is radiated to the glass substrate Sub0, only desiredregions of the substrate are exposed to a laser beam by using thealignment mark T9 as a registration mark, as illustrated in FIG. 13B.

Thereafter, alignment can be carried out in later steps based on thealignment mark T9 and an alignment mark (not illustrated) newly formedby patterning the thin crystal silicon film T2 a.

Then, the steps having been explained with reference to FIGS. 12C to 12Iare carried out as illustrated in FIGS. 13C to 13I.

Thus, there is fabricated a thin-film transistor having highperformances and reliability.

FIGS. 14A to 14I illustrate respective steps of a method of fabricatinga thin-film transistor. In this method, an alignment mark is formed atthe same time when a laser beam is radiated to a substrate.

First, a glass substrate Sub0 is washed to remove organic substances,metals and particles therefrom. Then, a substrate cover film T1 and athin silicon film T2 are formed on the glass substrate Sub0, asillustrated in FIG. 14A.

Then, the substrate Sub0 is washed to remove organic substances, metals,particles and oxides therefrom. Thereafter, the substrate Sub0 isintroduced into the apparatus for forming a thin film, in accordancewith the present invention.

Excimer laser XeCl having a wavelength of 308 nm is radiated to the thinsilicon film T2 as a laser beam L0 to thereby reform the thin siliconfilm T2 to a thin crystal silicon film T2 a, as illustrated in FIG. 14B.At the same time when the laser beam is radiated to the thin siliconfilm T2, a crystallized alignment mark T9 is formed in the thin crystalsilicon film T2 a, making use of a difference in quality between anexposed region and a non-exposed region.

After completion of laser radiation, an oxygen gas having a purity of99.999% or greater is introduced into the apparatus. The above-mentionednitrogen gas is used for preventing impurities from entering the thinsilicon film T2 a, prevent impurities such as hydrocarbon from reactingwith a ultra-violet ray around a laser introduction window, and preventimpurities such as hydrocarbon from sticking to a laser introductionwindow in burning.

The reason why crystallization is carried out at 700 Torr or greater isthat it is possible to prevent silicon evaporated by laser radiationfrom sticking to the laser introduction window.

After exhausting the gases, the substrate Sub0 is transferred to theplasma CVD chamber through the substrate-transfer chamber. Then, a firstgate insulating film T3 is formed at a substrate temperature of 350degrees centigrade, using silane, helium and oxygen gases as processgases, as illustrated in FIG. 14C. The first gate insulating film T3 iscomprised of a silicon dioxide film and has a thickness of 10 nm.Thereafter, the substrate is subject to hydrogen plasma treatment andannealing, if necessary.

Then, the thin silicon film 2 a and the first gate insulating film T3are patterned into an island by photolithography and etching, asillustrated in FIG. 14D. In this photolithography, the crystallizedalignment mark T9 acts as a registration mark.

Then, the steps having been explained with reference to FIGS. 12E to 12Iare carried out as illustrated in FIGS. 14E to 14I.

Thus, there is fabricated a thin-film transistor having highperformances and reliability.

While the present invention has been described in connection withcertain preferred embodiments, the advantages obtained by theaforementioned present invention is described hereinbelow.

As mentioned above, in accordance with the present invention, oxygen gashaving a high purity is introduced into a vacuum chamber just after alaser beam has been radiated to a silicon film for crystallization. As aresult, a natural oxidation film having a low concentration of impurityis formed on a surface of the silicon film. Since a surface of thesilicon film having been crystallized by laser radiation is quiteactive, contaminants readily adhere to the surface of the silicon film.However, the present invention makes it possible to prevent contaminantsexisting in the chamber from adhering to a surface of the silicon film.Accordingly, since a frequency of cleaning and maintenance of thechamber can be reduced, it is possible to reduce an efficiency ofoperation of the apparatus, ensuring enhancement of a total fabricationefficiency of a semiconductor device.

In addition, since it is possible to reduce an amount of carbon existingin a silicon dioxide film and an interface of a silicon dioxide film, itis possible to reduce current leakage in a thin-film transistor.

At the same time when oxygen gas is decomposed by a laser beam, siliconhaving adhered to a laser introduction window and having not beenoxidized by oxygen is heated. Hence, the thus heated silicon reacts withthe decomposed active oxygen to thereby form silicon dioxide, ensuringreduction in a transmission rate of a laser beam.

The present invention prevents contaminants such as metal and carbonfrom adhering to a surface of a silicon film formed on a substrate, andfurther prevents reduction in a transmission rate of a laser beam into alaser introduction window. As a result, it is possible to form aqualified interface or gate insulating film over the silicon film formedon a large-area substrate, ensuring that there is provided a fieldeffect transistor including an interface between semiconductor and aninsulating film which interface has a small interface level density,that is, having superior performance.

In addition, the present invention makes it possible to present asemiconductor device having the same characteristics of a conventionalsemiconductor device and being able to be formed at room temperature to600 degrees centigrade, even if a cheap glass substrate is used.

Furthermore, the present invention makes it possible to carry outmaintenance of the apparatus with an efficiency of operation of theapparatus being kept high.

The present invention provides the secondary advantages as follows.

First, the present invention provides an apparatus of forming a thinsemiconductor film which apparatus can omit a step of washing asubstrate with chemical and which apparatus has high stability.

Secondly, the present invention provides an apparatus which can carryout various steps, ensuring reduction in a total area of a factory.

Thirdly, the present invention provides a method and an apparatus bothof which are capable of keeping a washed surface of a silicon film cleanwithout chemicals.

While the present invention has been described in connection withcertain preferred embodiments, it is to be understood that the subjectmatter encompassed by way of the present invention is not to be limitedto those specific embodiments. On the contrary, it is intended for thesubject matter of the invention to include all alternatives,modifications and equivalents as can be included within the spirit andscope of the following claims.

The entire disclosure of Japanese Patent Application No. 11-201974 filedon Jul. 15, 1999 including specification, claims, drawings and summaryis incorporated herein by reference in its entirety.

What is claimed is:
 1. method of fabricating a thin-film semiconductordevice, comprising the steps of: (a) melting and recrystallizing atleast a surface of a thin semiconductor film formed on a substrate, in apressure lower than an atmospheric pressure or in inert gas atmosphere;(b) keeping said substrate in atmosphere including oxygen gas; and (c)forming an insulating film on said thin semiconductor film with saidsubstrate being kept in a pressure lower than an atmospheric pressure orinert gas atmosphere.
 2. The method as set forth in claim 1, whereinsaid atmosphere predominantly includes oxygen gas.
 3. The method as setforth in claim 1, wherein said atmosphere includes oxygen gas havingpurity of 99.999% or greater.
 4. The method as set forth in claim 3,wherein said atmosphere further includes hydrogen gas having purity of99.999% or greater.
 5. The method as set forth in claim 3, wherein saidatmosphere further includes nitrogen gas having purity of 99.999% orgreater.
 6. The method as set forth in claim 3, wherein said atmospherefurther includes inert gas having purity of 99.999% or greater.
 7. Themethod as set forth in claim 3, wherein a process gas used for formingsaid insulating film has purity of 99.999% or greater.
 8. The method asset forth in claim 1, wherein a process gas used in said method includeshydrocarbon (CnHm) species having a total concentration of 1 ppm orsmaller.
 9. The method as set forth in claim 1, whereinrecrystallization in said step (a) is carried out through laserradiation.
 10. method of fabricating a thin-film semiconductor device,comprising the steps, in sequence, of: (a) introducing a substrate intoa vacuous chamber; (b) introducing non-reactive gas into said chamber;(c) radiating laser beam to said substrate in said chamber; (d)introducing oxygen gas into said chamber; (e) exhausting saidnon-reactive gas and said oxygen gas until a pressure in said chamber isreduced down to a predetermined pressure; and (f) taking said substrateout of said chamber.
 11. The method as set forth in claim 10, whereinsaid non-reactive gas comprises at least one of a nitrogen gas, an inertgas, and hydrogen gas.
 12. The method as set forth in claim 10, furthercomprising, before exhausting said non-reactive gas and said oxygen gas,directing said laser beam to a window of said vacuum chamber, such that,said laser beam does not irradiate said portion of said substrate, whichwas previously irradiated.
 13. A method as set forth in claim 10,further comprising heating said substrate.
 14. A method of fabricating athin-film semiconductor device, comprising: introducing a substrate intoa vacuum chamber; introducing a non-reactive gas into said vacuumchamber, such that, a first pressure of 700 Torr or greater is attained;irradiating a portion of said substrate in said vacuum chamber with alaser beam; stopping of said introducing of said non-reactive gas intosaid vacuum chamber; after irradiating said portion of said substrateand said stopping of said introducing of said non-reactive gas,introducing oxygen gas into said chamber, such that, said vacuum chambermaintains said first pressure; and transferring said substrate from saidvacuum chamber into a processing chamber having a second pressure equalto said first pressure.
 15. The method as set forth in claim 14, furthercomprising: after said substrate has been transferred into saidprocessing chamber, directing said laser beam to a window of said vacuumchamber; and subsequently exhausting said non-reactive gas and saidoxygen gas until said processing chamber attains a predeterminedpressure less than said first pressure.
 16. The method as set forth inclaim 14, wherein said non-reactive gas comprises at least one of anitrogen gas, an inert gas, and a hydrogen gas.
 17. The method as setforth in claim 14, wherein after attaining said first pressure, saidnon-reactive gas is maintained at said first pressure in said vacuumchamber.
 18. The method as set forth in claim 14, further comprisingheating said substrate.
 19. The method as set forth in claim 1, furthercomprising forming said thin silicon film by using at least one of agaseous silane, being one of monosilane, disilane, or trisialane, argongas, hydrogen gas, and silicon tetrafluoride, on a substrate cover film.20. The method as set forth in claim 1, further comprising: removing aportion of said first gate insulating film and said thin crystal siliconfilm to expose said substrate cover film; and forming a second gateinsulating film on said first gate insulating film, said thin crystalsilicon film, and said substrate cover film.
 21. The method as set forthin claim 1, further comprising: forming a gate electrode above saidsecond gate insulating film; and doping a portion of said thin crystalsilicon film with ions by using said gate electrode as a mask.
 22. Themethod as set forth in claim 1, further comprising: forming aninterlayer insulating film above said portion of said thin crystalsilicon film that is doped, said first gate insulating film, said secondgate insulating film, and said gate electrode; and forming a contacthole through said interlayer insulating film to said portion of saidthin crystal silicon film that is doped.
 23. The method as set forth inclaim 1, wherein irradiating said thin silicon film with a laser beam islimited to a portion of said thin silicon film, determined by analignment mark formed on a layer disposed beneath said thin siliconfilm, that acts as a registration mark.
 24. The method as set forth inclaim 1, wherein irradiating said thin silicon film with a laser beam islimited to a portion of said thin silicon film, determined by acrystallized alignment mark formed in said thin silicon film, that actsas a registration mark.
 25. The method as set forth in claim 1, whereinsaid inert gas comprises at least one of nitrogen gas, helium gas, andargon gas.